This invention relates to intraocular lens implants.
An intraocular lens implant can be used to partially restore the sight in an eye whose natural lens has been damaged by injury or by a disease process.
Anatomical features of the healthy human eye are shown diagramatically in FIG. 1. Light reaching the eye 10 passes first through the cornea 12, the clear front part of the outer wall 11 of the eye. The cornea is a fixed focus lens that accounts for the majority of the focal power of the eye. Behind the cornea 12 the light passes through an aqueous solution 13, termed the aqueous humor, and then through the lens 14, which further focuses the light. Behind the lens 14 the light passes through a gelatinous substance 15, termed the vitreous humor, and then it reaches the retina 16, in which the light receptors are located, on the inner surface of the wall 11 at the rear of the eye.
In a healthy eye, the natural lens is pliable, and a musculature associated with the lens can by relaxation and contraction change the shape of the lens, and particularly its thickness. Changes in the thickness of the natural lens result in changes in its focal length, allowing the lens to accommodate, that is, allowing it to focus onto the retina a clear image of objects located at various distances from the eye. The closest point on which the eye can focus, termed the "near point", can be as close as about 7 cm in a healthy young adult, and becomes greater with age or as a result of certain disease conditions.
Suspended in the aqueous humor directly anterior to the lens 14 is the iris 18, a membranous diaphragm perforated by the circular pupil 20, through which the light passes to the lens. The iris responds to varying degrees of brightness of the incident light by expanding or contracting the pupil size to allow more or less light to pass through the lens. The pupil diameter of a healthy human eye varies from about 2 mm in bright light to about 7 mm in very dim light. In optical design terminology, the iris forms the entrance pupil of the eye. The cornea 12, pupil 20, and lens 14 are aligned with their centers generally on an optical axis O--O'.
Persons suffering from cataracts gradually lose their vision as the natural lens becomes opaque to light. The cataract sufferer's vision can be restored, to a degree, by surgical removal of the damaged natural lens and replacement of it with an intraocular lens implant.
The capacity of an intraocular lens implant to restore normal vision to the eye is substantially limited by the fact that the lens implant, unlike the natural lens, has a generally fixed focal length. As described above, the healthy lens is capable of accommodation so that objects at a distance to which the person's attention is at any moment directed are at that moment sharply in focus. An intraocular lens implant is typically made from an incompressible polymeric material and its shape cannot be changed by the focusing musculature of the eye, and consequently the implant is incapable of accommodation.
An improved intraocular lens implant can be made by forming a diffractive lens profile over the entirety of the posterior or anterior surface of a refractive lens body, producing a diffractive/refractive lens. The refractive component of the lens forms a clear image onto the retina of objects located approximately at a first distance from the eye, and the diffractive component forms a clear image onto the retina of objects located approximately at a second distance from the eye. As a result, a person fitted with such a diffractive/refractive "bifocal" intraocular lens implant can see a clear image of objects located both at distances within a close range and at distances within a more distant range.
The healthy natural lens can focus effectively all the incident light onto the retina for objects at any distance at or beyond the near point. Although a diffractive/refractive lens implant is an improvement over the wholly refractive lens implant in that it provides for a focused image of either close or distant objects, it delivers substantially less than all the incident light to a focus at the retina. In known diffractive/refractive lens implants substantially less than half the incident light may be properly focused for either near or far objects, and the remaining portion of the light is not properly focused on the retina for either near or far objects, resulting in a degradation of the quality of the perceived image. Such a diffractive/refractive lens configuration may also be subject to artificial color effects, that is, distant objects may appear more blue in color than they actually are, and near objects more red.